1. Field of the Invention
This present invention relates generally to the field of integrated circuit connectivity and, more specifically, to the field of plating contact structures upon bond pads.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In today's complex computer systems, speed, flexibility, and reliability in timing and control are issues typically considered by design engineers tasked with meeting customer requirements while implementing innovations which are constantly being developed for computer systems and their components. Computer systems typically include a variety of electrically interconnected integrated circuit (I/C) packages which perform a variety of functions, including memory and processing functions. Electrical interconnection of these I/C packages typically include numerous bond pads, which are structures that interface with the external connectors that join the assorted circuits. Typically, the external connectors that interface with the bond pads are either wires or solder balls depending on the mounting technique employed.
Whatever technique is employed, a conductive layer is typically disposed upon the recessed bond pads of the I/C package to provide an electrical contact surface for the solder or the wire. In the event solder balls or bumps are employed, the deposition of such a conductive layer is referred to as underbump metalization. Electroless deposition of nickel is typically used to form the conductive layer during the underbump metalization process and also for depositing a conductive layer in preparation for wire bonding.
Electroless nickel deposition is performed using a chemical bath containing nickel and stabilizers. The stabilizers control the manner in which nickel is deposited, often by enhancing the plating of large surfaces in preference to smaller surfaces. Controlling the amount and type of stabilizer therefore allows one to select which features are plated.
Due to the manner in which electroless nickel deposition is performed, the conductive layers formed on the bond pads tend to be shaped like mushrooms, spilling over the recessed bond pad and extending outwards. Since deposition typically occurs isotropically, the periphery of the conductive layer tends to continue expanding both upward and outward until deposition is halted. This “spillover” deposition necessitates that bond pads be spaced apart by a minimum safe distance to prevent inadvertent electrical contact between bond pads. The additional space necessitated by these spillover depositions adds unnecessary size to the I/C package or, alternately, prevents the attainment of more dense configurations of bond pads upon the I/C package. These effects prevent the optimum scaling of the I/C package from being achieved.
Additionally, the mushroom cap shape associated with the conductive layer is not optimal either for wire bonding or for solder ball techniques. The mushroom cap shape, while producing an acceptable wire bond, consumes an unnecessarily large surface area. Additionally, even with increased inter-pad spacing, the overflow increases the likelihood of incidental electrical interconnection between adjacent bond pads. For wire bonding, it would be preferable for the surface area presented by the conductive layer to correspond to the area actually needed for a successful wire bond and no more.
In the case of the solder ball or solder bump based techniques, the balls or bumps are disposed upon the conductive cap layer. The rounded surface of the conductive layer is not optimal for maximizing the shear strength of such connections. Instead, the surface area between the conductive layer and the solder structure is relatively minimal, producing less interface area to withstand shearing events. It would be preferable to construct conductive layers that minimize or eliminate such spillovers and increase the interface area available for solder ball contacts.
The present invention may address one or more of the concerns set forth above.